The present invention relates to a mechanism for gauging the catch of fish present within a trawl net.
In order to optimize trawl fishing it has been a long standing desire to be able to measure the amount of fish actually caught in the net, and to do this while the trawl is in actual operation. At the present time the usual procedure is to tow the trawl for about 4 hours at a time, while the actual launching of the trawl takes half an hour and the heave on board again another half hour. Considering the non-fishing part of the operation being that long, it is usually desirable to heave a full trawl net each time.
On the other hand, the net must not be too full before heaving. This applies in particular to deep sea operation, since the swim bladder of the caught fish will greatly extended during the heave to the surface. Many cases are known where the net has burst due to the heavy pressure resulting in loss of catch and great damage to the equipment. Proper means for gauging the catch during tow will indicate the correct time for starting to heave.
Such a gauge mechanism may also indicate whether the net has been damaged or possibly torn to pieces. If no catch is indicated in spite of ample prior echo-sounding or sonar registrations, the net may no longer be intact and the fish may be lost. Should the net turn out to be in proper condition, the original echo indications were presumably not of fish, but might have been sound reflections from plankton, roe, or some other type of reflective material.
Such mechanism to gauge the catch will also serve to facilitate adaption of the size of the catches to the processing facilities on board, thus avoiding excessive storage of raw fish.
Numerous procedures have been proposed for gauging the extent of fill or the size of the catch within the net of a trawl. One such procedure includes echo sounding of the trawl net from a transducer mounted near the front of the trawl, in the direction of the far end of the net, thus obtaining echo signals from the catch. The distance indicated by such signals will then be a measure of the size of the catch. Another method is based upon the idea that fish caught in the net will activate pressure switches mounted at strategic locations in the net. Further, it has been suggested to utilize the variable resistance between two electrodes mounted within the net as indication of the extent of fill. Detection of reflection of a beam of light has also been proposed to indicate whether fish are present in the net.
It is not known if any of the above proposals have been reduced to practice in a manner suitable for commercial fishing operations.